This invention relates to vibration damping. More particularly, though not exclusively, it relates to the damping of vibrations in aerofoil blades for gas turbine engines.
Gas turbine engines commonly include an axial-flow turbine that comprises at least one annular array of radially extending aerofoil blades mounted on a common disc. Each aerofoil blade is provided with a circumferentially extending platform near to its radially inner end so that the platforms of adjacent blades cooperate to define the radially inner circumferential boundary of the gas flow path over the blades.
In operation, there is a tendency for the gas flows over the aerofoil blades to cause the blades to vibrate to such an extent that some degree of damping is required. A commonly used design of prior art damper is axially elongated and essentially wedge-shaped in cross section, with two friction surfaces at its radially outer end. These friction surfaces are angled at approximately 60xc2x0 to the radial direction of the blades and subtend an angle of approximately 120xc2x0. The damper is located between two adjacent blades, radially inward of the blade platforms. The radially inner faces of the blade platforms are designed to subtend the same angle as that subtended by the damper friction surfaces. In operation, centrifugal forces tend to draw the damper radially outwards so that its friction surfaces are brought into planar contact with the angled faces on the radially inner surfaces of the platforms. Any vibration of the blades will result in relative movement between the platforms of adjacent blades, and hence in sliding movement between the blade platform faces and the damper friction surfaces. The work done in overcoming the frictional forces associated with this sliding movement dissipates the vibrational energy in the blades and reduces the vibration.
One drawback of this design of damper is that as the relative positions of adjacent blades change as a result of blade vibration, the angle subtended by the blade platform faces may no longer be the same as that subtended by the damper friction surfaces. The surfaces are then no longer in planar contact; the damper will tend to tilt or rock rather than sliding, and the damping effect is lost.
Various designs have been proposed to overcome this problem. EP 0509838 discloses a wedge-shaped damper having raised pads on the two friction surfaces of the damper. The raised pads are located so as to reduce tilting of the damper and keep the raised pads in planar contact with the platform faces. U.S. Pat. No. 5,478,207 discloses a damper which is generally wedge-shaped but which has an offset centre of mass, intended to improve the stability of the damper and to maintain planar contact between the damper friction surface and the blade platform face.
Although these designs of damper address the problem of loss of planar contact, they share a further drawback, in that they are not effective for all modes of vibration. The classical theories of bladed disc vibration identify three types of vibrational modesxe2x80x94blade flap modes, edgewise modes and torsional modes. In an idealized situation, a perfectly tuned bladed disc (i.e. one in which all the blades have the same natural frequency) with a synchronous excitation (e.g. from upstream vanes) would give rise to a single vibration mode with a defined inter-blade phase angle. The smaller the number of vanes, the lower would be this phase angle. In a real situation, however, the blades will not all have the same natural frequency, so the relative blade motions will be complex and will encompass different types of vibrational modes.
It is therefore an object of the present invention to provide an improved damper, which will provide more effective damping in all vibrational modes.
According to the invention there is provided a blade-to-blade vibration damper for a gas turbine engine, the damper including a first friction surface for contacting a first face associated with a turbine blade and a second friction surface for contacting a second face associated with an adjacent turbine blade, said first and second friction surfaces and said first and second faces being planar, said first friction surface and said second friction surface being convergent, the closest-spaced ends of said first friction surface and said second friction surface being spaced apart by a distance at least as great as the maximum circumferential gap between the radially outer ends of said first face and said second face, the angle subtended by said first friction surface and said second friction surface being smaller than the angle subtended by said first face and said second face; wherein the mass of the damper is disposed such that the centre of mass of said damper lies in a plane bisecting the angle subtended by said friction surfaces.
Preferably the damper is substantially wedge-shaped in cross section.
Preferably said closest-spaced ends of said first friction surface and said second friction surface are joined by a convex, curved surface.
Preferably the difference between the angle subtended by said first friction surface and said second friction surface and the angle subtended by said first face and said second face is approximately 10xc2x0. In a particular preferred embodiment of the invention the angle subtended by said first friction surface and said second friction surface is approximately 110xc2x0, and the angle subtended by said first face and said second face is approximately 120xc2x0.